Great Ape Juvenile Development and Digestive Physiology

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Humans and many non-human primate species have long periods of juvenile development, but the evolutionary underpinnings of these patterns are not fully understood. Orangutans (Pongo pygmaeus), which have a slow life history and extreme fluctuations in their food supply, are a good species for testing hypotheses about factors that may favor slow juvenile development during evolution. In this project, multiple methodologies will be applied in a long-term study of orangutans in Gunung Palung National Park, Indonesia, to determine how food availability affects juvenile growth and development, and the extent to which juvenile and adult orangutans differ in foraging behaviors, diets, and overall ability to extract energy from foods. The project provides research opportunities for undergraduate and graduate students and increases scientific capacity near the research site. The project runs parallel to the principal investigator's conservation program that helps conserve orangutans through education, alternative livelihoods, adoption of customary forests, and sharing of research with local authorities. Using social media, website, Facebook, twitter, monthly newsletter, and weekly radio programs, the project educates local and world-wide audiences about the importance of orangutan conservation and the application of research to solving conservation problems. The project team members also will develop a special Girl Scout patch program to teach skills related to scientific leadership, field biology and the study of animals to girls in the upper elementary/middle school range, the highest risk group of children for dropping out of STEM subjects. Understanding what constrains juvenile development and the ability of the growing organism to translate energy into growth and maintenance, before the onset of reproduction, is central to our understanding of animal life history, including the study of human evolution. It has been argued that human evolution has been marked by selection for a slower life history. Existing comparative data on great ape development are limited, and based primarily on captive animals living in energy-rich environment, thus likely experiencing accelerated growth. This study sheds light on the concept of ecological risk aversion by looking at both developmental and ecological influences on digestion. The investigators will use behavioral observation, physiological measures, and data on diet and available food resources to determine: 1) if juvenile orangutans are at greater ecological risk than adults, whether juveniles are able to extract less energy from their food, the role of masticatory force in breaking up food into smaller particles, and how body size determines gut passage rates and how much energy is extracted from food; 2) how juveniles and adults differ in their foraging behavior, diet composition and overall energy expenditure; 3) how juvenile development impacts maternal behavior and physiology; 4) at what rate juvenile orangutans grow and whether this differs depending on food availability; and 5) how juvenile orangutan physiology changes with development and depends on food energy intake. If digestion is impeded when consuming high fiber fallback foods during periods of poor food availability, we can better understand the compensatory behaviors that both juveniles and their mothers take to offset caloric and nutrient deficiency. Because of their smaller body size, incomplete gut biome, and masticatory immaturity, juveniles are predicted to be more vulnerable to periods of food shortage. Additionally, if larger-bodied animals of the same species are able to process fiber more efficiently than smaller-bodied animals, this has could have implications for how the evolution of body size impacts the ability to achieve a higher quality diet.